The present invention relates generally to a distributed or decentralized plant control system and more particularly to such a distributed control system for a plant in which various control apparatuses are distributed in the field where various plant machines, devices, instruments, tools or the like (also referred to as electromechanical units, machine units or field apparatus in more general terms) to be controlled by the control apparatuses are installed.
In the present state of the art, the control systems for various plants inclusive of thermal power plants and others tend increasingly to be realized by resorting to the use of digital technology. A typical example of such digital plant control systems is disclosed in Japanese Patent Application Laid-Open No. 21503/1986 (JP-A-61-21503). This known control system is implemented in the form of a centralized control system in which various electromechanical units installed in the field within a plant are placed under the control of a control system installed at the central station of the plant. For more particulars of the whole arrangement of this known control system designed for a thermal power plant, reference may be made to the above mentioned publication.
In the centralized control system, any abnormality taking place in the control system would exert adverse influence to the plant operation over an extensive range. Under the circumstance, there arises a tendency for decentralizing the plant control system. An example of such decentralized or distributed plant control system is disclosed in JP-A-58-66111. In this known distributed plant control system, a high-rank or host computer is installed in a central control station room in which operators are resident, while field control apparatuses are installed in the field in the vicinity of the electromechanical units to be controlled thereby, wherein the host computer and the individual field control apparatuses are interconnected through a medium of a loop-type data highway system.
A plant of any type may be divided into three major stations for convenience' sake of consideration. One of them is a central control station including a central control room in which a control and monitoring console or panel for supervision, a host computer, a general control system for controlling and monitoring the whole plant in a consolidated fashion and the like are installed. The second station is constituted by the so-called field control station disposed in association with a plurality of the electromechanical units to be controlled. The third station is a so-called electrical equipment room in which power supply equipment is accommodated for supplying electric energy to the control system and the computer in the central station as well as to the electromechanical units, operation terminals for operating the electromechanical units and the operation terminal controllers installed in the field by tapping from a power source external of the plant (outdoor power supply source) or from an indoor power supply source in the case of an electric power plant.
In the known control system disclosed in JP-A-58-66111 briefed above, the control system covering the central control station and the field control stations is certainly implemented in the form of the distributed or decentralized control system. It is noted however that decentralization is not realized for the power supply system intervening between the electrical equipment room and the field stations. Usually, the electrical equipment room accommodates therein buses for tapping electric power from an external power supply source, transformers for lowering the source voltage to appropriate voltage levels, circuit breakers for interconnecting the buses, switchgears provided on the load basis, a control center for controlling the on/off operation of the circuit breakers and the switchgears. The electrical equipment room of this nature is inherently ill-fitted for decentralization of the control system. As the consequence, there arise problems mentioned below.
(1) For effectuating the closing/opening control (on/off control) of the switchgears from the central control station, process signals available from the loads installed in the field are required in addition to the on/off command signals from the host computer installed in the central station. To this end, a signal transmission path must be established not only between the electrical equipment room and the central control station but also between the former and the field stations, which will neccessarily involve an enormous expenditure inclusive of the signal cable cost and installation cost. Besides, since the signal transmission lines and the power transmission lines are installed in juxtaposition, reliability of the control will be degraded due to disturbance by generation of noise and other causes, not to speak of complexity involved in providing the separate lines or cables for the power and signal transmissions.
(2) Even when the control apparatuses are distributively installed in the field, the central control station or room will have to be provided which is designed for accommodating such instruments, tools, display units, recorders and others as required for monitoring and controlling operations of various electromechanical units constituting the plant in order to supervise and control the plant operation in a consolidated manner. Consequently, when an electromechanical unit installed in the field has to be run for trial, serviced or inspected for maintenance or for other purposes, the field operator in charge must every time consult the operator resident in the central control station room to obtain appropriate commands and information from the central station, which is of cource considerably inconvenient.